As more and more devices gain networking capabilities, the need for a user to manage these multiple devices arises. Such a work item has been undertaken in 3GPP under the scope of collaborative session management. Here, a number of user's devices that registered with IMS services can cooperate with each other in a session with different media flows. A collaborative session is a multimedia session with multiple UEs involved and collaborative with each other. Controller UE manages media on controllers by interacting with an Application Server. Any request from controller must be authorized by controller before taking into action. Usually one media flow is controlled by just one controller.
Due to the single controller configuration (for a specific flow) in a collaborative session, there exists a problem when controller is lost due to uncontrollable reasons like UE breakdown, battery exhausted, UE out of coverage, unstable signal, etc. There are also situations when controller wants to change itself to passive-control mode or leave collaborative session temporarily. These cases may happen when controller doesn't want to be interrupted each time a controller makes a change, or when no controller propose any request for a long time. Here passive-control mode means that the controller UE opts to be in auto-control mode or temporarily gives the control to Application Server. That is, the controller UE stays passive by setting up rules that make decisions on certain trigger scenarios or assigning responsibility to other nodes such as Application Server or Controller UEs.
In latest 3GPP TS23.237, it is indicated that SCC AS releases all Access Legs participated in a Collaborative Session when controller is lost.
The problems of this approach are that the controllers are always forced to terminate the session without knowing what has happened, and they cannot continue or resume even if the user wants to continue and is willing to pay.
Another possible approach is to allow the SCC AS to transfer the Collaborative Session control to another UE involved in the Collaborative Session and belonging to the same subscription”, if the controller is lost [Non-patent document 4].
This method makes a step to solve this problem but it doesn't specify how to select the successive controller and what happens if other UEs are under different subscription. Obviously, some better solutions are needed to solve the problem for controller wants to change to passive-control mode, which is inevitable when the operator deployed the collaborative session service.